Homeostasis and FeedbackHomeostasis: processes that maintain the relatively stable internal environment that keeps an organism aliveFeedback: process that allows the body to sense changing internal conditions and respond to them

How Feedback Works

• A STIMULUS occurs Examples are:

A change in blood pressureA change in body temperatureAn injury to the bodyEtc……………

• A RECEPTOR picks up the stimulus Examples:

Receptors in blood vessels that sense being stretched

Receptors in the brain that sense blood temp., CO2 level, sugar level….

Damage to cells that release proteins that are normally in the cell.

• A message is sent to the CONTROL CENTER (the brain) or directly to an EFFECTOR.

The control center will send a response message to an Effector.

• EFFECTOR: A tissue or organ in the body that produces some type of action in response to the message from the receptor or the control center.

Examples: A gland that begins to release a hormone or stops

releasing a hormone.A muscle that begins to contract faster or slower.

RESPONSE – maintains homeostasis

Two Types of Feedback

Positive Feedback1. Definition: Change

toward initial conditions2. When does this happen?

When some critical process must be completed

Negative Feedback1. Definition: Change away

from initial conditions2. When does this happen?

Corrects a situation that has moved away from homeostasis.

Example of Positive FeedbackBlood Clotting

Watch an animation of blood clotting:

http://www.mhhe.com/biosci/esp/2002_general/Esp/folder_structure/tr/m1/s7/trm1s7_3.htm

Example of Negative FeedbackLow Blood Pressure

1. Dehydration leads to low blood pressure2. Low blood pressure detected by

baroreceptors on the heart3. Those baroreceptors send message to brain4. The brain sends a message to both the blood

vessels and the heart5. Arteries constrict and heart rate increases6. Result: Normal blood pressure

Fill in a chart over negative feed back and watch an animation over low blood pressure

http://www.johnwiley.net.au/highered/interactions/media/Foundations/content/Foundations/homeo2c/bot.htm

Identify the following examples as positive or negative feedback and answer the questions on

the Homeostasis Worksheet

The control of sugar levels in the blood:

Childbirth As a baby moves towards the birth canal, it presses against the .pressure

receptors in the muscular part of the uterus. These receptors evoke a release of oxytocin from the brain (and maybe also the placenta). When the oxytocin reaches responsive receptors in the muscles of the uterus it will increase muscular tension thus increasing stimuli to the pressure receptors. This goes on as "labor" until the pressure is relieved: the baby is born -- oxytocin is no longer evoked and labor contractions cease.

Baby’s head presses Message sent to Oxytocin released Contractionspressure receptors the brain. from pituitary Increase in the uterus wall “time for baby to gland

be born”

More pressure from baby on uterus wall.

The Control of Body TemperatureCooling mechanisms – External blood vessels dilate and more blood flows to the skin losing heat, sweating beginsWarming Mechanisms – External blood vessels constrict and more blood stays in the internal part of the body keeping internal organs warm, shivering begins